The term "reversible fuel cell" denotes a device capable of operating both to generate current from the reaction between a fuel and an oxidant and to electrolyze the combustion products.
The electrodes concerned in the present invention, are for use in devices operating at elevated temperatures, in particular within the range 600.degree. to 1000.degree. C., to generate an electric current from gaseous fuels and oxidants or, inversely, to dissociate electrolytically into their components the products resulting from the combustion of such fuels. These electrodes are made of porous ceramic materials capable of conducting electrons and ions and, by reasong of their porosity, they are permeable to gases. These gases are either reactive gases or reaction products. As examples of such gases there may be mentioned H.sub.2, CO, NH.sub.3, CH.sub.4 and other hydrocarbons, H.sub.2 O, O.sub.2, N.sub.2 and air. When the device operates as a generator the anode is the negative electrode to which the combustible gases migrate and the cathode is the positive electrode where the oxidant gases are reduced by electrons which have passed through the external electrical circuit. In the electrolytic mode, the polarities are the same, but the designation of the electrodes (cathode and anode) is inverted.
In fuel cells, the electrodes are separated by a solid electrolyte which is impermeable to gases, and is referred to subsequently as the substrate. The latter, which must provide for the migration of the O.sup.2- ions produced at the positive electrode in the direction of the negative electrode where they are discharged, is usually composed of metal oxides, examples of which have been described in Swiss Patent Specification CH 594 292.
The component materials of the electrodes, detailed examples of which have also been disclosed in Swiss Patent Specification CH 594 292, generally comprise solid porous mixtures of metal and metal oxide powders in the form of agglomerated fine particles (cermets). By way of examples of such metal powders there may be mentioned powders of Ni, Co, Cu, Ag, Au and Pt. As examples of metal oxides there may be mentioned thoria, hafnia and zirconia, stabilized in the cubic phase by oxides such as MgO, CaO, Y.sub.2 O.sub.3 and the rare earth oxides. Mention may also be made of oxides employed by reason of their good electronic conductivity, for example the oxides of Sn, In and Bi. Moreover, in order to improve the electronic conductivity of stabilized zirconia the latter may be doped with metal oxides, notably with uranium and cerium oxides for the electrodes in contact with the fuel (anodes in battery mode and cathodes in electrolyzer mode) and the oxides of uranium and of praseodymium for the electrodes in contact with the oxidant (O.sub.2).
The materials constituting the electrodes may also contain other components intended to improve their mechanical strength and, more particularly, to ensure their compatibility with the solid electrolyte with which they are associated, notably from the point of view of expansion in the course of temperature variations.
The foregoing indications relating to the composition of the electrodes are given purely by way of illustration, because, generally, the invention does not relate to the choice of the materials used in the electrodes but, in spite of an exception referred to later, more specifically to their nature and their structure which derive essentially from the manufacturing process.
In particular, these electrodes were hitherto manufactured by various processes among which there may be mentioned spraying onto the substrate of particles of the electrode constituents by mechanical means or in a blowpipe-flame or plasma, vapor-phase deposition (C.V.D.), and high-vacuum evaporation-condensation.
However, all these processes suffer from drawbacks generally linked with the extreme conditions (high temperatures, deformations, etc. . . . ) which prevail during their use. Thus they usually lead to deposits of irregular shape and thickness, and compositions which are difficult to reproduce. Moreover, the versatility of these processes is low, and they lend themselves poorly, if at all, to the deposition of well-controlled, variable-composition electrode layers for modern fuel cells.